This invention relates to a method and apparatus for magnetizing material, and more particularly, to an apparatus and method for magnetizing material in such a manner that the magnetic pole transitions are custom-shaped.
Direct current brushless motors and other electrical devices employ an annular shaped article of magnetized material having a relatively thin wall thickness, which is generally referred to as a ring magnet. Ring magnets can be composed of any suitable material, such as, SmCo.sub.5, Alnico, barium ferrite or a plastic NdFeB. To create magnetized areas of alternating north and south magnetic poles on a ring magnet (including transition regions of no magnetization between each adjacent north and south magnetic pole) magnetizers of various configurations are employed.
The distribution of the magnetic poles and transition regions on the ring magnet have a direct effect on motor performance due to the spatial distribution of the permanent magnetic flux fields interacting with the stator and stator windings of the motor.
The spatial distribution of these permanent magnetic flux fields affects the amount of cogging torque [.sup.dRm /.sub.d.alpha. ] of a brushless motor. This is an undesirable torque that is directly proportional to the change of the magnetic reluctance R.sub.m (the reluctance through the stator seen by the magnetic pole's permanent magnetic flux fields of the ring magnet) with respect to the rotor position .alpha..degree.. If the width of the magnetic poles and transition regions do not match the stator geometry, then the magnetic reluctance R.sub.m seen by the magnetic poles will change with different rotor positions .alpha..degree. (.sup.dRm /.sub.d.alpha. .noteq.0), causing the rotor to cog. This cogging torque is a major parasitic component in the output motor torque, thereby increasing noise and vibration and decreasing motor efficiency. Conversely, if the width of the magnetic poles and transition regions can be effectively controlled to match the stator geometry, then the magnetic reluctance R.sub.m seen by the magnetic poles will remain constant at different rotor positions .alpha..degree. (.sup.dRm /.sub.d.alpha. .noteq.0), thereby causing the motor to run more smoothly.
The internal torque generated by interaction of a winding current and the permanent magnetic flux fields of a brushless motor is not a constant torque in practical motor applications. The internal torque is directly proportional to the product of the back EMF generated by the motor and the current generated by the power supply, which both have a direct effect on the amount of the variation in the internal torque, referred to as a ripple torque. In turn, the back EMF is directly proportional to the permanent magnetic flux fields. Therefore, by controlling the spatial distribution of the permanent magnetic flux fields the back EMF has a direct impact on the ripple torque. The present inventor has discovered that changing the transition regions between the magnetic poles by changing the impedance in the circuit of the magnetizer, and by reducing or increasing the current in the secondary winding of the magnetizer, can change the back EMF wave-shape. One such example is to optimize the back EMF wave-shape to a flat square wave-shape or to a sinusoidal wave-shape for reducing the ripple torque depending upon the motor power supply used.
Tsukuda, U.S. Pat. No. 4,614,929, discloses a method for the magnetization of an annular magnet by placing a magnetic core on opposing sides of an annular magnet. Magnetizing members of each core are spaced-apart circumferentially in conformity with the shape of the annular material and are opposed perpendicularly to each other across the annular material. Energizing the magnetic members on each magnetic core creates magnetic flux fields which are mutually reinforcing, thereby creating magnetized regions of alternating polarity on the annular magnet. This mutual reinforcement makes it difficult to control the width of the transition regions between adjacent magnetic flux fields. Further, because of oversaturation of the magnetic cores, the magnetic flux fields are not confined to regions within the magnetic poles; therefore, leakage occurs through the slots making control of the width of the transition region even more difficult.
A. K. Littwin, U.S. Pat. No. 3,417,295, discloses a magnetizer for magnetizing certain cup-type units used in generators and motors. The magnetizer includes an outer casing having a surrounding wall and one or more magnets on the inner surface of the wall. The magnetizer has pole elements on the outside and magnetizing heads on the inside which are mutually opposed as to polarity, thus establishing an intense flux through the magnet material. With the mutually opposed polarity, the resulting magnetic flux fields reinforce each other making the width of the transition region between adjacent poles difficult to control.
Other magnetizers are shown in the following U.S. Pat. Nos: 5,093,595, 3,678,436, 3,335,377, 4,575,652, 3,585,549, 4,692,646, 3,158,797, and 3,317,872, but none provide any capability for controlling the width of the transition area between adjacent poles of an annular magnetically permeable material.